The instant invention is in the field of epoxy resin compositions. More specifically, the instant invention relates to compositions for hardening or curing epoxy resins, and especially, epoxy resins useful in powder coating applications.
Due to their physical and chemical properties such as high resistance to chemical attack and good adhesion to various substrates, epoxy resins are useful in the preparation of coatings. The epoxy resins can be applied, for example, from organic or aqueous solutions onto a variety of different substrates or applied in a powder form to be cured into a film coating by heating.
Conventionally, in preparing a powder coating using an epoxy resin, particles of a solid epoxy resin are blended with particles of a hardener such as a compound containing one or more reactive phenolic hydroxyl groups or one or more amine groups, which diffuse with and react with the epoxide groups of the epoxy resin to form a hard thermoset coating. For example, an epoxy powder coating has been prepared in situ on a suitable substrate by applying a mixture of particles of solid epoxy resin with particles of a solid epoxy hardener composition having more than one phenolic hydroxyl groups and a suitable accelerant or catalyst to a substrate and subsequently heating the coated substrate to effect the reaction between the epoxy containing component and the phenolic hydroxyl containing component. If desired, the mixture can also contain dyes, pigments and flow control agents.
Although the adhesion of the coating prepared from the above-described epoxy resin compositions to a substrate is generally sufficient, the adhesion of the cured epoxy resin coating to various metal substrates such as a mild steel substrate when the coated substrate is subjected to humid conditions has been improved by the use of various chemical modifications to the epoxy resin compositions as disclosed in Japanese Patent Applications Kokai Nos S59-24762 and H2-105817; U.S. Pat. Nos. 4,678,712; 7,001,938; and U.S. Patent Application 2004/0147690. Despite the above-mentioned advancements in the art, there remains a need to further improve the adhesion and processing economics of the cured epoxy resin coating to various metal substrates such as a mild steel substrate when the coated substrate is subjected to humid conditions.